I have a bathroom fan that used to be vented by flexible aluminum
ducting through the soffit. But I was getting ventilated soffits to
work with an attic ridge vent and it seemed ill advised to dump all
that humid air right under the soffit vents. So I switched to a roof
vent.

When I ran the ducting for the roof vent I used rigid aluminum ducting
instead of the flexible stuff, since I always read that the rigid
ducting has a lower resistance to air flow. I used a 1 foot section of
flexible ducting right at the fan exhaust port and then about 1 foot
more of rigid ducting, and then an 80 degree angle bend up to the roof
(about 2 ft) and then a 40 degree angle bend or so to connect to the
roof cap.

The fan itself is a quiet fan. And with flexible aluminum ducting out
through the soffit it was quiet. You basically couldn't hear the fan
if you weren't in the bathroom. But with the rigid aluminum ducting it
is much louder. What can I do to quiet the rigid duct? Or is the only
quiet solution to rip out the rigid duct and use a flexible duct
instead?